I can't solve this jerking problem!?!?

[tech9]

I have had this problem with one of my 77 280z's for over a year now and no one seems to know what is wrong! I have posted a lot about it on a local zcar group, and on hybridz and no luck fixing the issue.

Here is a summary of the problem and what I have tried to resolve it.

Under very light load, the car will jerk violently as if there is a huge amount of drive train slop, at first it was just slightly, but its gotten so bad I'm afraid to drive it. I can get it to happen in reverse, all gears, but not under acceleration or in neutral/clutch disengaged. A typical time of it happening is your cruising in 3rd/4th and you come to a stoplight and hit the brake, and as your slowing down, it will jerk very rough till you hit the clutch. Kinda hard to explain without being there :\

Here is what i've done to the car: (note, the jerking problem was happening before all this)

T5 swap (using t5 driveshaft/flange)

replaced tranny mounts of course with swap

fidenza alum flywheel

new clutch/pp

new ujoints in driveshaft

new ujoints in halfshafts

new differential (unless somehow I have 2 bad diffs?)

new diff mount

replaced mustache bar bushings, rear swaybar bushings

etc etc.

The most progress I've made is there was slack in the diff cover bolts because previous owner had the wrong washers, so i torqued all of the mustache bar/diff bolt nuts correctly and there is no more noise from this, but it didn't solve the problem.

Not really sure what else to look at, someone said motor mounts, they look OK, but I might change them just to be sure.

Any help or suggestions will help!

thanks! -Joe

[Darrel]

Did you have the problem with the nissan transmission?

oops, I guess you did.

[ConchZ]

My 73 did this when I first got it. Another symptom was that the starter sometimes had difficulty cranking the engine, usually when the engine was cold. This cranking difficulty triggered me to look at the timing. It was set for an initial timing of about 30. Not sure why the PO had done this, although maybe he did it because the vacuum advance doesn't work. I backed the timing down to about 16 (mechanical advance does work, and brings it up to about 30 at around 2500 rpm). This cured the hard cranking, and then also cured the weird bucking under a light load. I'm not sure I completely understand why it bucked with an initial timing of 30, so if someone explains it, I'd appreciate the schooling. Anyways, check your timing.

[theramz]

I would try to duplicate the probnlem by raising the rear of the car and while in first gear, motor off, rotate one wheel back and forth as hard as possible and look at the various parts in the drive rain. It could be elongated holes in the diff cover or mustach bar. Bad mounts etc.

[JimmyZ]

Are you sure that the throttle linkage isn't doing it? A bad engine mount could cause the engine to rock a bit which would twist the engine in such a way as to act like you were playing with the throttle.

If it won't do it under a firm decel (completely letting off gas) or if it's diminished under a firm decel then I'd definitely look into the rocking engine theory. (Mounts)

2c

Jim

[tech9]

Thanks for the help.

The 5speed was not bad, but I needed an upgrade I guess. Needed the 5speed nissan in my other Z actually.

I'm having some timing problems, but I threw my dissy in the trash and going with EDIS (installing all the MS stuff now) so we'll see if that is the issue.

Also going to change the throttle linkage using a 240sx TB, and a custom linkage, so we'll see if the motormounts/linkage was any of the issue.

I had it on stands and running a while back, and I just could not find anything wrong, but I'm willing to do it again, as soon as the engine runs again (its in a million pieces right now).

Thanks again for the help guys, I will update soon.

[angry_dave]

I had a 93 ford bronco that would buck under light acceleration, deceleration because the egr solenoid went out. I replaced this solenoid after it stopped when i unplugged it, and the problem was fixed. just seemed kinda similar. i dont think it is in the drive train, new tranny, diff, ujoints, driveshaft, bushings. not the drive train

[Z Babe]

Tech9, have you double-checked those U-joints to make sure they aren't lose? I had the same symptoms in one of my Zs, and that was the problem.

[adamr]

This may be a shot in the dark, but have you checked your air flow meter?